252 



MINERALOGY. 



[Sect. VIII. 



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4 



coniuion terms, may be called a horizoutal as a vertical 

 manner. They are not due to the drippings of water 

 charged with the matter of the minerals in solution, such 

 as are often seen in fissares, the resulting deposits beip<y 

 more or less crystalline according to conditions ; on the 

 contrary, the particular modes of occurrence to which we 

 allude, seem more the result of crystalline deposits from 



ons (fil 







so acted upon, that projections in a given line, more es- 



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pecially if composed of certain substances, received these 

 deposits — in fact much in tlie same manner as substances 

 in solution may be thrown down by well-known methods 

 when galvanic action is employed. We have before us 



an illustrative specimen from the Consols Mine, Gwennap, 

 Cornwall, in which large crystals of quartz are on the 

 one side covered by crystals of sulphnret of iron, and on 

 the other by crystals of copper pyrites. Cases where 

 crystals of one substance abundantly occur on one side of 

 prior-formed crystals of another substance, and not on the 

 reverse or opposite sides, are sufficiently common, and best 

 seen in the fissures or mineral veins themselves. 



Although when exposed to the action of weather, the 

 minerals which may be found in veins or fissures, open on 

 the feces of cliffs, are not very often (except when of sub- 

 not easily injured) in a good state of preservation, 

 they show that such minerals are found in the vein, so 

 that if time and opportunity permit, some unexposed part 

 of the vein may be broken into. Success may not, cer- 

 tainly, always attend such a search, for it is curious to 



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rve how very locaL even in the same vein, the occur- 



rence of a particular mineral may be* 



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